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Furnace and Combustion Chamber.

347

finished by chipping. The bolts being clamped, their holes are
also countersunk, being first rimered to ensure exact correspond-
ence. The rivet holes both at front and back of boiler are next
drilled by placing the latter on a cradle, which allows the flat
plates to stand vertical, and face four drill standards supporting
horizontal drills on suitable saddles. The boiler joints being
truly level, the rivet holes may be easily drilled, as well as the
stay holes in the back, the latter being made to tapping size.

IX.  The Furnace Tubes (22 and   23) are  usually obtained
rolled, flanged, and cut to correct shape, an allowance being left
at front end for turning.     They may be flanged, however, under
the machine in Fig. 290, as shewn at 24, using special dies.   Mark
off all the flange holes, as at 23, and drill all those at £, one in
every six at <r, but none at the corners d.

X.   Combustion Chamber Throat Plate.—This is flanged to the
shape shewn at 25.    A rectangular plate being procured, the
centres of the furnaces are found as at 26, a hole trepanned, and
the flanging of the throat done at one heat, as at R, Fig. 290.
The rest of the plate is lined as at 27 and the corners cut, the
sides *, f, g, and h being flanged progressively until the whole fits
a cast-iron block or template.    This is of course an operation
involving great care.    Now the portions 2$a and 25^ are sawn
out, finishing the plate with the exception of the  taper ends,
which are drawn out by heating and hammering on the cast-iron
block.    After milling the flange edges, the rivet holes 23^, con-
necting with the Fox tube, are marked from the latter, and drilled
separately; and the flange holes carefully spaced out by reference
to the top corners and the furnace centres, but only one in every
six drilled now, and none through the taper portion.

XI Combustion Chamber Back Plate (28).—This must be
lined out and flanged progressively to fit a cast-iron block, and
the flange edge then milled. The stay holes are drilled in
position.

XII. Tfae Cowr PLties for the Combustion Chamber are now
edge^planed, rolled, and beat bot with hammer, until thfey exactly
fit the flanged plates, as shewn in Fig. 311. There are three of
these plates, one for the roof, and one for each side; and the
holes already drilled in the flanged plates must be traced through

B B